The present invention relates to a process for stabilization of the viscosity of wood pulps, that have been treated with ozone or ozone/oxygen during a bleaching sequence, at a level of materials corresponding to those that are obtained by the conventional chlorine bleach processes.
Bleaching of wood pulps at the present time takes place predominantly with the use of chlorine or chlorine-containing bleaching agents. However, oxygen-containing bleaching agents such as oxygen, ozone or hydrogen peroxide are being used increasingly. This is due to the undesirable pollution of waste water caused by release of chlorinated compounds.
It has been shown that the chlorine requirement for a conventional final bleach can be considerably reduced by the use of oxygen for predelignifications. However, the use of chlorine cannot be avoided completely.
Delignification with just oxygen or hydrogen peroxide produces only limited lignin degradation rates. If very drastic delignification conditions are used in the oxygen step, there is irreversible damage to the wood pulp.
The combined use of oxygen and ozone is necessary for intensification of the delignification. However, ozone is a very reactive and simultaneously nonselective bleaching agent. Thus, side reactions, such as the oxidation of wood pulp, cannot be prevented even with low ozone charge amounts. In this case, the resulting carbonyl groups elevate the sensitivity of wood pulp toward alkaline degradation. The alkaline extraction following the ozone treatment, which takes place in acidic medium, leads to a cleavage of cellulose chains and thus to a reduction of the viscosity and strength of the wood pulp in comparison to materials bleached with the use of chlorine. O. Kordsachia and R. Patt in the journal, Holzforschung 42, 203-209 (1988), report that the reduction of the average polymerization values caused by ozone treatment can be at least partially suppressed by the addition of sodium borohydride. However, this is possible only at low ozone dosages (0.5%) which yield modest brightnesses (86 (ISO)).